Overview
The town of Colstrip, in southeastern Montana, is the site of four thermal generating plants, divided into Colstrip Units 1&2 and Colstrip Units 3&4. Colstrip Units 1&2 are twin 333 gross MW plants that have been in operation since the mid-1970's. Colstrip Units 3&4 are twin 805 gross MW plants that have been in operation since the mid-1980's. The entire generating plant complex is referred to as the "Colstrip project". In operation, a thermally beneficiated low rank coal (TBLRC) trade named SynCoal.RTM. is delivered by truck from the ACCP demonstration facility to the Colstrip project for use in Unit 2 on a daily basis. The SynCoal.RTM. is stored in a silo, and delivered pneumatically to three (3) of the Unit 2 coal mills at a continuous rate up to about 40 tph.
In conjunction with the U.S. Department of Energy under its Clean Coal Technology program, Western SynCoal LLC, a non-regulated indirect subsidiary of the Montana Power Company, is conducting a full-scale commercial demonstration of a patented technology which enhances Powder River Basin coal. The technology reduces moisture and sulfur and substantially increases BTU content (e.g., from 8,600 BTU/lb to 11,700 BTU/lb). These alterations to the raw coal result in a thermo beneficiated low rank coal trade marked as SynCoal.RTM., a product which is drier, and cleaner-burning. The facility for producing SynCoal.RTM. is called the Advanced Coal Conversion Process plant (ACCP), and is located in Colstrip at the Western Energy Company (WECO) mine, and operated by WECo personnel.
SynCoal.RTM. is delivered by truck from the ACCP demonstration facility to the Colstrip project for use in Unit 2 on a daily basis. The SynCoal.RTM. product is stored in Units 1 and 2, and delivered pneumatically to three (3) of the Unit 2 coal mills at a continuous rate of up to about 40 tph.
Invention Demonstration Description
Using a single truck with tandem trailers hauling approximately 50 tons of SynCoal.RTM. per load, the delivered load from the ACCP is discharged onto the new unloading hopper which incorporates two (2) new 24" diameter screw conveyors and a new bucket elevator. The material is first fed from the trailer to the unloading screw conveyor positioned parallel to the truck, which in turn feeds the transfer screw conveyor perpendicular to the truck. The transfer screw conveyor in turn feeds a totally enclosed bucket elevator at a rate of 200 TPH. SynCoal.RTM. is transferred from the 135' high bucket elevator to the southern-most lime silo. The modified lime silo, fitted with a bin vent dust collector, holds approximately 600 tons of SynCoal.RTM. product.
Located within the existing modified silo building, the silo bottom is fitted with a three-way distribution manifold for mass flow of SynCoal.RTM. discharged into each of the three (3) rotary airlock feeders. One rotary airlock feeder corresponds with fuel supply to each of three (3) Unit 2 coal mills, through a 6" diameter pneumatic feed line. One rotary airlock feeder supplies fuel to the pneumatic pipe ending at mill #2A, another rotary airlock feeder supplies mill #2B, and the last rotary airlock feeder supplies #2D. A pneumatic operated knife-gate valve is located above each rotary airlock feeder for service of the equipment. Each 6" schedule 40 pneumatic feeder line is piped from the rotary airlock feeder to a 10" diameter expansion elbow located on the existing mill 12" diameter fuel down comer.
Each of three (3) pneumatic feeder pipes is supplied compressed air from each of three (3) positive displacement blowers sized to supply 1400 SCFM. The blowers are located in a new pre-engineered steel building, which in turn is located to the east of the silo building.
Each rotary airlock feeder is fitted with a venting wye and piped in such a manner as to facilitate the entrance of the product into the feeder pockets. The vented gas is piped to a new baghouse, which discharges vented gas to the atmosphere, and routes solids to the bucket elevator inlet chute.
The feed rate flow of SynCoal.RTM. flow to any single pulverizer will range between 2 and 20 TPH through each of the three (3) rotary airlock feeder feeders. The total capacity of the SynCoal.RTM. system with three (3) rotary airlock feeders running at their maximum speed is approximately 60 TPH, which is less than one third of the Unit 2 fuel requirements.
The rotary airlock feeders are proportionally controlled from the Unit 2 Control Room. Control of the feeders is effected through rotational speed rate (RPM) corresponding to a calculated mass flow rate. The control allows variation of the flow of SynCoal.RTM. by variable frequency drives. The new SynCoal.RTM. Feed Control System is configured to control SynCoal.RTM. feed while interfacing with appropriate signals from the existing Unit 2: 7300 Burner Control System and the Furnace Safeguard Supervisory System (FSSS). The SynCoal.RTM. Feed Control System is software programmable in order to provide an efficient means of changing the system operating characteristics. The SynCoal.RTM. Feed Control System consists of a PLC (GE) with I/O equipment, a workstation computer (MMI) and a monitor located in the Unit 2 Control Room Control Board.
The SynCoal.RTM. Feeders are initially started at minimum speed. Opening of the Silo Gate occurs after startup of the associated SynCoal.RTM. Feeder. Once started, the SynCoal.RTM. feed rate control may be placed in automatic. While in automatic, changes in the Feeder Master signal will divide the change between the SynCoal.RTM. feed rate and the raw coal feed rate equally on a BTU basis. The Silo Gate closes when either the SynCoal.RTM. feeder, or the raw coal feeder, or the associated mill or the entire Unit is tripped. During a normal shutdown, the Silo Gate will close and the associated SynCoal.RTM. feeder will shutdown after a time delay in order to purge the feeder and its upstream piping.
The pipe arrangement is such that any pulverizer can be removed from operation, while each of the other two pulverizers are fed from the SynCoal.RTM. pneumatic system. Each of the pipe bends are wear resistant to protect against abrasion. The piping and equipment from the silo to the pulverizer feed piping are designed to withstand a 50 psig dust explosion pressure per FM recommendations. In addition, each pneumatic line is fitted with a Deflagration Isolation System, designed per NFPA-69 to close the pneumatic line off in two directions to prevent propagation of an explosion event. Sensors mounted in the downstream pulverizer piping, and the upstream pneumatic transport piping send a signal to quickly close two deflagration isolation valves in the event an explosion event is detected.
A small membrane type nitrogen separator (approximately 1,700 SCFH) supplies 97%+pure nitrogen to the top of the silo continuously, thus preventing air infiltration into the SynCoal.RTM. product. A connection from the Unit 2 CARDOX system to the silo allows the potential to flood the silo with carbon dioxide in the event combustion is detected within the silo. Explosion (deflagration) vent panels are located on the silo and bucket elevator, based on NFPA-68 guidelines.
The SynCoal.RTM. Control includes continuous silo level and CO (carbon monoxide) concentration indication, in addition to trouble from either the Nitrogen system, or the Rotary Airlock Feeder vent baghouse.